Police and local authorities are the main organisations to work in partnership with. You could also consider involving your local NHS bodies, which may consider helping action to tackle speeding because of the public health implications.
It's worth contacting operators of any vehicle fleets in your area, such as buses or businesses, to ask them if they have a policy to deal with any notifications by CSW schemes that their drivers have been seen speeding. Many operators already publicise a 'how am I driving scheme' so are likely to want to be seen as responsible employers. If operators of buses or goods vehicles fail to take action against speeding by their employees, they could have their Operator Licences withdrawn - see the managing freight and lorries section.
There are also links to consider with the eco-driving section, as if a critical mass of drivers are driving more carefully, it becomes harder for others to speed.